Heat exchangers for air conditioners may be constructed of a U-shaped copper tube bent like a hairpin and fins made from aluminum or aluminum alloy plate.
Accordingly, a copper tube used for the above type heat exchanger requires suitable conductivity, formability, and brazing properties.
HCFC (hydro-chlorofluorocarbon)-based fluorocarbons have been widely used for cooling media used for heat exchangers such as air conditioners. However, HCFC has a large ozone depleting potential such that other cooling media have been selected for environmental reasons. “Green refrigerants”, for example, CO2, which is a natural cooling medium, have been used for heat exchangers.
The condensing pressure during operation needs to be increased to use CO2 as a cooling media to maintain the same heat transfer performance as HCFC-based fluorocarbons. Usually in a heat exchanger, pressures at which these cooling media are used (pressure of a fluid that flows in the heat exchanger tube) become maximized in a condenser (gas cooler in CO2). In this condenser or gas cooler, for example, R22 (a HCFC-based fluorocarbon) has a condensing pressure of about 1.8 MPa. On the other hand, the CO2 cooling medium needs to have a condensing pressure of about 7 to 10 MPa (supercritical state). Therefore, the operating pressures of the new cooling media are increased as compared with the operating pressure of the conventional cooling medium R22.
Due to the increased pressure and to some loss of strength due to brazing in some tube forming processes, conventional copper materials have to be made thicker thereby increasing the weight of the tube and therefore the material costs associated with the tube.
What is needed is a heat exchanger tube that has high tensile strength, excellent processability and good thermal conductivity that is suitable for reducing the wall thickness, and therefore, the material costs, for ACR heat exchangers and that is suitable for withstanding high pressure applications with new “green” cooling media such as CO2.